Heater power supplies



April 1958 J. R. BECKWITHY 2,831,151

HEATER POWER SUPPLIES Filed Jan. 2, 1957 I 4T729/PA/E Unite HEATER POWER SUPPLIES John R. Beckwith, Mexico City, Mexico, assignor to Raytheon Manufacturing Company, Waltham, Mass, a corporation of Delaware Application January 2, 1957, Serial No. 632,112

6 Claims. (Cl. 315-96) heaters) of such tubes in series and to place the series across the power supply lines. It is desirable to utilize directly the potential available from ordinary commercial electric supply systems to energize the series-connected heaters, because such supply systems are standardized in the matter of frequency and supply potential, and such practice eliminates the interposition of converting apparatus such as transformers. Insofar as electronic apparatus is involved, a further advantage of the series connection is that where the design is such that a plate voltage supply transformer is not used, total elimination of power supply transformers is effected, which is important in lowlevel or high gain amplifiers where magnetic flux leakage from a power transformer may cause hum to be introduced into signal carrying circuits. Total elimination of power transformers in the low voltage power supply may be achieved by employing voltage multiplying rectifier circuits directly connected to the power lines to furnish B+ voltage.

At the present time, the use of series-connected heaters is accompanied by serious disadvantages. One of the more pressing problems is that the thermal inertia of heaters may vary widely among different tube types and among tubes of the same type made by different manufacturers, so that, where the heaters of a number of such tubes form a series-string, an excessive voltage drop may occur across an individual heater during the warm-up period. In the series arrangement, the warm-up period is especially critical because the resistance of a heater undergoes substantial change with rising temperature. As a consequence, during the warm-up period, one or more of the heaters in a series-string may encounter a voltage drop of such magnitude as to cause the heater to burn out. Moreover, after the heaters of the string are brought to a steady-state operating condition, an excessive voltage may continue to be applied to one or more of the heaters unless the tubes are carefully selected to have similar heater characteristics. Because of the diversity in heaters among the various tube types, it is common practice today to compensate for variations in heater characteristics by shunting one or more of the heaters in a series-string by a current-carrying resistor. This method of compensation is adequate, but it is undesirable because it increases the heat which must be dissipated. In addition, the temperature-resistance characteristic of the shunt resistor often is not identical with that of the shunted heater, further complicating the matching of the several heaters, and further unbalancing the voltage distribution across the series-string.

One solution to the problem of absorbing the thermal shock when current is first applied to the heaters is to utilize series resistors having negative temperature cotates Patent Patented Apr. 15, 1958 eflicients of resistance. Such special resistance devices are termed thermistors and are effective in limiting the initial current permitted to flow through the seriesstring. The principal disadvantage of the thermistor is that it continually consumes power and thereby aggravates the heat dissipation problem. A secondary disadvantage is that such special devices are expensive.

A major disadvantage which inevitably accompanies the series-string heater arrangement is that the failure of one heater interrupts the current through all the other heaters in the string so that every tube in the series becomes inoperative. The defective tube, as a consequence, cannot be identified by visual inspection alone, as would be the case were the heaters connected in parallel. Moreover, Where several electron tube circuits are related solely through the series connection of their heaters, the failure of one heater results in the several circuits all. becoming inoperative.

Having recognized the disadvantages associated with the series-string heater arrangement, the present invention entirely overcomes or greatly reduces those disadvantages. The invention employs an inductor having a coil wound upon a soft iron core to enhance its inductance, the coil being divided. into sections by a number of taps. The inductor is placed across the power lines, and each of the coil sections is connected in paraliel with a different heater of the series-string. taps on the coil are spaced so that each coil section has an induced voltage which is equal to the rated voltage of the heater with which it is in parallel. This arrangement insures the correct starting voltages being applied to alt the heaters. During the critical warm-up period, the stabilizing influence of the inductor prevents any one of the heaters in the series-string from having an inordinate voltage applied to it. As the warm-up time is of short duration, usually the fractional part of one minute, the current drawn from the inductor by the heaters does not cause appreciable heating in the inductor coil. Once the warm-up period is passed, the inductor is virtually floating across the power lines, and the power consumed by the inductor is limited substantially to the exciting power which, in a properly-designed inductor, is very minor. The exception to this occurs when the steady-state operating characteristic of one or more of the heaters does not exactly match that of the other heaters so that power is taken from the inductor to accomplish an electrical matching. If for any reason one of the heaters in the series-string should become open-circuited, all of the other heaters will continue to operate with minor changes in their individually-applied voltages because the inductor will thenfunction as an autotranslormer. In the latter circumstance, the defective tube can be readily identified by visual inspection.

The invention, both as to its embodiments and method of operation, will be better understood by reference to the detailed description set forth below when taken in conjunction with the accompanying drawings wherein:

Fig. l is a schematic view of the simplest embodiment of the invention; and

Fig. 2 is a schematic View of a supply incorporating the invention.

With reference to Fig. 1, there is indicated an inductor 1 having a coil wound upon a soft iron .core 2. The inductor 1 is connected at terminals 3 and 4 to power lines 5, 6 which are energized from an alternating current source '7. Also connected across the power lines is a series-string of heaters, here represented as resistors 8, 9, i9, i1, and 12. For expository purposes, it will be assumed that the heaters are identical insofar as voltage and current ratings are concerned and that the sum of the rated voltage drops across the heaters is equal to the line voltage. Therefore, because of the series conneclow-voltage power tion of heaters, each heater after the warm-up period will draw the same amount of current and the voltage drops across the individual heaters Will be equal. The inductor is: tapped at various points so that the coil is divided into sections 13, 14, 15, 16 and 17. Section 13 is connected in parallel with heater 8, and sections 14, 15, 16, and

17 are shunted across heaters 9, 10, 11 and 12, respectively. Section 13 is proportioned so that the voltage induced across thatsection is equal to the rated voltage for which heater 8 is designed. In like manner, the voltages induced across the other sections of the coils are matched to the rated voltages of the other heaters in the series-string. As long as the series-string of heaters is continuous from. terminal 3 to terminal 4 and each of the heaters draws its rated current, the inductor merely floats across the power lines and requires only a meghetizing current, i. e. only sufficient current to induce current only from the power line, and, since no current is required from the inductor by the heaters, the power dissipates within the inductor is minute. In comparison to the size of a transformer required to energize the same heaters in the conventional parallel connection, the physical size of the inductor is appreciably smaller. Where the prevention of the introduction of hum into signalcarrying circuits is an important consideration, it is possible, because of the small size of a suitable inductor, to substantially reduce magnetic flux leakage by overdesign of the inductor so that very low flux density obtains in the soft iron core.

Fig. 2 diagrammatically depicts an efiicient power supply employing a transformer to provide B+ voltage and in which the transformers primary winding is utilized to stabilize the heaters of a series string. A number of electron tubes having heaters 25, 26, 27, 28, and 29 are arranged so that the heaters are in series across power supply lines 5, 6. Each of the heaters is connected in parallel with a section of primary winding 21 of transformer 20 in a manner identical to that described in connection with Fig. l. The transformer 20 includes a soft iron core 22 on which are secured primary winding 21, secondary winding 23, and a winding 24 for supplying heating current to the filament of rectifier tube 30. The anodes of the rectifier tube are connected to opposite ends of secondary winding 23 and that winding is center-tapped at 31 for connection to a filter (not shown). Winding 24 is also center-tapped to complete the circuit through the filter. Since the series heaters 25 to 29 draw power from the primary of the transformer only during the short warm-up period or when one of the heaters becomes defective, the transformer 20 may be designed solely to meet the demand for B+ and filament heating current. The circuit configuration of Fig. 2 enables a power supply transformer of materially reduced bulk to be employed because separate windings for the heaters 25 to 29 are eliminated. A further reduction in the bulk of transformer 20 can be achieved by employing dry rectifiers, such as the selenium or copper-oxide type, in lieu of vacuum tube rectifier 30, so that winding 24 becomes superflous and may be entirely eliminated without degrading the performance of the B+ power supply.

The invention disclosed herein materially enhances heater longevity and performance when used in the series arrangement and eifectually reduces or removes the disadvantages associated with that arrangement.

This invention is not limited to the particular details ,4 of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. A heater power supply circuit comprising an inductor having a coil for connection to a source of alternating potential, a plurality of heater elements connected in series, means shunting each series heater element across a section of said coil having an induced voltage equal to the rated voltage of the shunted heater element.

2. A heater power supply comprising an inductor having a coil connected to a source of alternating potential, a series-string of heater elements connected across said alternating potential source, and means shunting each of said heater elements across a different section of said coil having an induced voltage equal to the rated voltage of the shunted heater element.

3. A heater power supply circuit comprising an inductor having a coil energized from a source of alternating potential, a plurality of heater elements connected in series, said series being connected to said alternating potential source, said coil being tapped at various points to define a plurality of coil sections, and means shunting said series heater elements across said coil sections to connect each heater element in parallel with a'section having an induced voltage equal to the rated voltage of theshunted heater element.

4. A power supply circuit comprising a transformer having a primary winding for connection to a source of alternating potential and a secondary winding for connection to a rectifying device, a plurality of heater elements connected in series, said series being connected to said source of alternating potential, and means shunting each series heater element across a section of said primary winding having an induced voltage equal to the rated voltage of the shunted heater element.

5. A power supply circuit comprising a transformer having a primary winding for connection to a source of alternating potential and a secondary Winding for connection to rectifying apparatus, a plurality of heater elements connected in series, said series being connected across said source of alternating potential, said primary winding being tapped at various points to divide said primary winding into a plurality of sections, and means shunting said series heater elements across the sections of said primary winding to connect each heater element in parallel with a section having an induced voltage equal to the rated voltage of the shunted heater element.

6. A power supply circuit comprising a transformer having a primary winding connected to a source of alternating potential and a center-tapped secondary winding connected at its ends to separate anodes of a fullwave rectifying vacuum tube, a separate winding on said transformer for supplying filament heating current to said tube, a plurality of vacuum tube heater elements connected in series, said series being connected across said alternating potential source, taps on said primary winding defining a plurality of winding sections, and means shunting said series heater elements across said winding sections to connect each heater element in parallel with a section having an induced voltage equal to the rated voltage of the shunted heater element.

References Cited in the file of this patent UNITED STATES PATENTS 1,503,709 Pruden Aug. 5, 1924 1,864,482 Carpenter June 21, 1932 2,020,786 Klinkhammer et al. Nov. 12, 1935 

